In my imagination the cars are kinda like swiss knives, which, before the race, already got mounted with various weapons, and the pickups are only enabling them for the pilots to use (like in the ). Let's say, minding my logic that the car carries two batteries. One is a fixed, EMP protected energy battery to keep up everything related to driving, like the control panel, servo motors, gyroscopic seat, pilot health monitor and the like, and the other one is a fast rechargeable one for the pickups, like the KERS in Formula 1. I assume there that most pickups do not use up much energy from that battery. Maybe maintaining the Aegis or the Gattlers gun motor could be an expection from that, minding the recently available repertoire. Since the EMP is an electromagnetic discharge, it is most effective when it's energy source is loaded to the maximum before releasing it all. That's when the charge-up sequence comes in. The faster you go, the faster it charges up.
Altought i noticed one or two flaw in my theory:
While the charge-up is in progress, you shouldn't be able to use any other pickup in that short meantime (and after that for a little while, since the other pickups need some energy to build up aswell).
If you didn't used up any of the built up kinetic energy in the race, so the battery is on the maximum, the EMP should be instant.
And that brings the question, if there would be indeed that kinda battery, how would you know it's charge level? Maybe a plus index for it?
Well, that's all for my looney fantasies for now